1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer, a recorded image displaying device, or a facsimile for developing an electrostatic latent image formed on an image bearing member to form a visualized image using an electrophotographic system, an electroststatic recording system, or the like.
2. Related Background Art
Heretofore, an image forming apparatus such as a copying machine using an electrophotographic system or an electroststatic recording system is provided with a developing apparatus for visualizing an electrostatic latent image formed on an image bearing member such as a photosensitive drum by sticking developer to the electrostatic latent image, thereby forming a developer image (toner image). Developing systems for the developing operation carried out by the developing apparatus are roughly classified into a one-component developing system and a two-component developing system. In this case, a description will hereinafter be given with respect to a conventional example adopting the two-component developing system using a two-component developer containing toner and a carrier.
The basic construction of the developing apparatus includes a developer container for containing therein a developer, and an opening portion which is provided in a portion of the developer container opposed to an image bearing member. Also, a rotatable developer carrying member having irrotational magnetic field generating means built therein, e.g., a developing sleeve or a developing roller is installed in the opening portion with its peripheral surface being exposed. The developer carrying member bails out the developer contained in the developer container through a function of the internal magnetic field generating means, and feeds the developer up to a surface of an image bearing member through its rotational operation while the developer carrying member bears the developer. The developer is moved onto the surface of the image bearing member by applying a developing bias voltage to the image bearing member. In such a manner, a developing operation is carried out.
In a case of a conventional developing apparatus which is adapted to contain the two-component developer and which adopts such a basic construction, firstly, as a first example, a developing apparatus 102 is adopted in many cases, which has a construction as shown in FIG. 8 in which a first conveying screw 5 and a second conveying screw 6 as circulation means for conveying the two-component developer contained in a developer container 2 while agitating the two-component developer to cause the two-component developer to circulate within the developer container 2 are horizontally disposed.
In the developing apparatus 102, an opening portion is provided in a portion of the developer container 2 for containing therein a developer opposed to an image bearing member (not shown), and a developing sleeve 8 as a rotary member is provided in the opening portion. Then, a magnet roller 8a as magnetic field generating means is built in the developing sleeve 8 so as to be fixed against the rotation of the developing sleeve 8. Then, the first conveying screw 5 located nearer an image bearing member (not shown) of the two circulation means 5 and 6 provided as developer agitating members (conveying members) inside the developer container 2 is used to supply the developer to the developing sleeve 8 and to collect the developer which has passed through a developing portion as an opposing portion through which the developing sleeve 8 and the image bearing member are opposed to each other. In addition, the second conveying screw 6 is used to mix and agitate the developer collected from the developing sleeve 8 and a newly supplied developer with each other.
On the other hand, in recent years, in an image forming apparatus such as a copying machine or a printer using the electrophotographic system, in order to attain the space saving, a requirement for miniaturization of the apparatus main body has increased. In particular, in an image forming apparatus using a full-color system, a demand for miniaturization has been strong since a plurality of developing apparatuses are used.
In order to solve this problem, as a second construction obtained by improving the first construction shown in FIG. 8, a developing apparatus 103 having a construction shown in FIG. 9 as described in Japanese Patent Application Laid-open No. H05-333691 is designed. A feature of the developing apparatus 103 shown in FIG. 9 is such that in addition to the basic construction including the above developing sleeve 8, two conveying screws 5 and 6 as circulation means for a developer are vertically disposed on upper and lower sides, respectively.
More specifically, the developing apparatus 103 includes a developer container 2 which contains therein a developer, and a developing sleeve 8 as a developer carrying member which is provided in an opening portion of the developer container 2 opposed to a photosensitive drum 10. Then, a developing chamber 3 and an agitating chamber 4 which are separated through a partition wall 7 are vertically formed on a side opposite to that of the opening portion within the developer container 2. First and second conveying screws 5 and 6 as circulation means for agitating and conveying the developer to cause the developer to circulate within the developer container 2 are installed in the developing chamber 3 and the agitating chamber 4, respectively. The first conveying screw 5 feeds the developer contained in the developing chamber 3, and the second conveying screw 6 feeds toner which is supplied from a toner supply port (not shown) to an upstream side of the second conveying screw, and the developer which is already contained in the agitating chamber 4 while it agitates the toner and the developer within the agitating chamber 4, thereby unifying the toner density in the developer.
As described above, the vertical agitation type developing apparatus 103 shown in FIG. 9 has an advantage in that since the developing chamber 3 and the agitating chamber 4 are vertically disposed on the upper and lower sides, a horizontal occupancy space may be small. Thus, for example, even a color image forming apparatus using a tandem system or the like in which a plurality of developing apparatuses are horizontally installed in parallel with each other can be miniaturized. In addition, the supply of the developer to the developing sleeve 8 is carried out in the developing chamber 3 provided on the downstream side in a rotation direction of the developing sleeve 8, and the collection of the developer is carried out in the agitating chamber 4 provided on the upstream side. In such a manner, since the supply and collection of the developer to and from the developing sleeve 8 are carried out in the separate containing portions. As a result, the new developer is supplied in a state of being uniformly mixed with the collected developer to the developing sleeve 8. This can contribute to the high image quality promotion.
The miniaturization of the developing apparatus 103 has been realized by adopting the vertical agitation type. However, in recent years, the development for further promoting the high image quality and the long life has been advanced in addition to the miniaturization for the developing apparatus using the two-component developing system. In particular, first of all, in order to attain the long living of the developing apparatus using the two-component developing system, it is necessary to adopt a construction adapted to prevent the developer from being compressed and to prevent the toner degradation and the carrier degradation (carrier spent)
Referring now to FIG. 9 for example, a place where the developer is compressed within the developer container 2 is a portion in which a layer thickness of the developer attracted onto the developing sleeve 8 is regulated, i.e., a layer thickness regulating portion as a portion opposed to a blade-like regulating blade 9 for example which is provided in the opening portion of the developer container 2. Normally, with such a construction of the developing apparatus, a developer layer thickness regulating magnetic pole of a magnetic roller 8a for bearing the developer regulated by the regulating blade 9 on the developing sleeve 8 is located on an upstream side in the rotation direction of the developing sleeve 8 with respect to the regulating blade 9 in the vicinity of the regulating blade 9. Thus, the developer attracted onto the developing sleeve 8 by the developer layer thickness regulating magnetic pole is compressed between the developing sleeve 8 and the regulating blade 9 on the inner side of the developer container 2.
Then, in order to weaken the compression of the developer between the developing sleeve 8 and the regulating blade 9 on the inner side of the developer container 2, it is effective to weaken a force by which the developer thickness regulating magnetic pole in the magnetic roller 8a attracts the developer onto the developing sleeve 8, i.e., a force Fr vertically acting on the developing sleeve 8. In order to attain this, it is necessary to construct a magnetic pattern in which the magnetization of the carrier in the developer is reduced, i.e., a force for rubbing a toner image obtained on an image bearing member 10 in the developing portion through the development is weakened to realize the high image quality promotion, whereby the lines of magnetic force from the developer layer thickness regulating magnetic pole are hard to come into the adjacent poles and they come out in a radial direction of the developing sleeve 8 as much as possible.
As one method of reducing the magnetization of the carrier in the developer, while adopted as a third construction in the developing apparatus 103 as well shown in FIG. 9, a developing method is proposed in which a repulsive magnetic field is formed by repulsive magnetic poles of the magnet roller 8a within the developing sleeve 8 provided inside the developer container 2, and one of the repulsive magnetic poles is used as the developer layer thickness regulating magnetic pole (refer to Japanese Patent Application No. H09-316478).
When the magnetic poles having the same polarity are located side by side to form the repulsive magnetic field in accordance with this method, the lines of magnetic force from the respective magnetic poles come out vertically to the surface of the developing sleeve 8. In this case, a rate of change in magnetic flux density vertical to the surface of the developing sleeve 8 is small. As a result, the force for attracting the developer onto the developing sleeve 8 is reduced, and the degree of compression of the developer is reduced accordingly.
However, when the construction of the conventional developing apparatus using the two-component developing system, i.e., the construction is adopted in which one magnetic pole of the repulsive magnetic poles is used as the developer layer thickness regulating magnetic pole, there is a possibility that screw pitch-like density unevenness is generated in a trailing end portion of a recording material in which a solid image, especially, a black solid image is formed in a conveying direction.
A mixture ratio between the developer which has the reduced density of the toner (having the image history) and which is moved to the developer layer thickness regulating magnetic pole after being peeled off by the repulsive magnetic field and the developer supplied to the developer layer thickness regulating magnetic pole portion of the developing sleeve after being agitated and conveyed by the conveying screw changes with a rotation period of the conveying screw in a longitudinal direction of an image area, thereby generating that phenomenon.
In addition, that phenomenon is apt to generate when an agent surface of the developer in the vicinity of the developing sleeve is relatively low and the conveying screw is disposed in the vicinity of the developer layer thickness regulating magnetic poles. Moreover, the above phenomenon is also apt to generate when the magnitude of the magnetization of the magnetic carrier is reduced. This reason is that when the magnetization of the carrier is small, the developer becomes magnetically insensitive to the magnetic field and the developer after completion of the development is readily moved to the developer layer thickness regulating magnetic pole without being peeled off by the peeling-off magnetic poles.
Then, in addition to the basic construction, the vertical agitation type construction as the second construction, and the construction as the third construction in which the repulsive magnetic poles including the layer thickness regulating magnetic pole are provided in the magnetic field generating means built in the developer carrying member in the developer container, a developing apparatus having a plurality of developer carrying members disposed therein was designated as a developing apparatus having a fourth construction. In a developing apparatus 101 shown as an example thereof in FIG. 10, there are disposed two developing sleeves, i.e., a developing sleeve 8 opposed to a photosensitive drum 10 on an upstream side in the rotation direction of the drum and a developing sleeve 11 opposed to the photosensitive drum 10 on a downstream side in the rotation direction of the drum. The developing sleeves 8 and 11 constitute a first developing portion X1 and a second developing portion X2 in the portions opposed to the photosensitive drum 10, respectively. In the developing apparatus 101, the screw pitch-like density unevenness can also be made inconspicuous. This effect is offered based on a function in which even when the screw pitch-like density unevenness is generated in the developing sleeve 8 on the upstream side, the screw pitch-like density unevenness is reduced through the development by the developing sleeve 11 disposed on the downstream side.
However, even when the developing apparatus has a plurality of developing sleeves, the screw pitch-like density unevenness cannot be perfectly erased.
Under the circumstances in which in recent years, there has been required the performance capable of coping with graphic images, such as a presentation document having high image duty, other than characters and graphs, there is a demand of preventing the screw pitch-like density unevenness from being generated.